The effect of impurities of light elements C, N, O on the velocity of motion of the crystallization front in fcc metals Ni and Ag was studied by the method of molecular dynamics. The dependences of the crystallization rate on the concentration of impurities are obtained. It is shown that the presence of impurities leads to a significant slowdown in the velocity of motion of the crystallization front in metals. Braking of the crystallization front by the impurity atoms is associated with the local deformation of the crystal lattice that they cause, and, as a rule, the greater this deformation, the stronger the impurity atoms inhibit the crystallization front.

No abstract

Методом молекулярной динамики проведено исследование влияния упругой одноосной деформации вдоль различных направлений на скорость миграции границ наклона с осями разориентации <100> и <111> в ГЦК металле на примере никеля. Одноосная деформация в модели задавалась в начале компьютерного эксперимента путем изменения соответствующих межатомных расстояний вдоль одной из осей. В работе получены зависимости скорости миграции при температуре 1700 К от угла разориентации. Показано, что большеугловые границы наклона <111> и <100> мигрируют приблизительно с одной и той же скоростью, тогда как малоугловые границы <111> мигрируют примерно в два раза быстрее границ <100>. Полученные зависимости скорости миграции границ от величины одноосной деформации почти во всех случаях оказались немонотонными и имели максимум при значении растяжения примерно 1%. При дальнейшем увеличении растяжения миграция замедлялась, что объясняется, по всей видимости, уменьшением поверхностного натяжения границ и, соответственно, движущей силы вследствие конечной сорбционной способности границ зерен по отношению к свободному объему. При упругом сжатии в большинстве случаев наблюдалось монотонное снижение скорости миграции, что обусловлено снижением при сжатии свободного пространства и уменьшением подвижности атомов на границе. Отличная зависимость была получена для границы <111> 30° при сжатии вдоль оси разориентации – в этом случае скорость миграции не падала, а, напротив, возрастала. Данная аномалия, вероятно, связана со снижением в результате сжатия вдоль оси разориентации энергии активации расщепления парных зернограничных дислокаций, которое играет важную роль в механизме миграции границ <111>.

The interaction of hydrogen atoms with nanocrystalline palladium and nickel in the work was studied by the molecular dynamics method. The nanocrystalline structure of palladium and nickel was created in the model by crystallization from the liquid state at the presence of several specially introduced crystalline embryos. After solidification, the calculation blocks, in addition to the crystalline phase, contained grain boundaries and triple junctions of grain boundaries. The interactions of metal atoms with each other were described by the multi-particle Cleri-Rosato potential constructed in the framework of the tight-binding model. Morse potentials were used to describe the interactions of hydrogen atoms with metal atoms and with each other. The parameters of Morse potentials were calculated from the experimental data of theabsorption energy, the activation energy of the above-barrier diffusion of hydrogen in a metal (at normal and high temperatures), the binding energy with a vacancy, dilatation. According to the results obtained in the present work, at a high concentration of hydrogen (the concentration of 10% from the metal atoms was considered), the hydrogen atoms combine into aggregates, which are formed predominantly near the surface of the metal. The aggregates contained, as a rule, several dozen hydrogen atoms and had low diffusion activity. The binding energy of hydrogen atoms with these aggregates was greater than with the metal crystal lattice or grain boundaries in it. In palladium, hydrogen aggregates were formed farther from the surface than in nickel. Apparently, this is due not so much to the relatively low energy of hydrogen absorption by palladium (–0.1 eV) in comparison with nickel (0.16 eV), but rather to the difference in lattice parameters of the metals under consideration: 3.89 Å for Pd and 3.524 Å for Ni. For the same reason, conspicuously, hydrogen aggregates in a pure crystal lattice were more often observed in Pd than in Ni. In Ni, aggregates, as a rule, were formed in defect areas containing an excess free volume: near the free surface, in grain boundaries and in triple junctions.

Effect of deformation along various directions against migrating  boundary on migration rate of edge boundaries with <100> and <111>  misorientation  axes  in  nickel  was  studied  by  means  of  molecular  dynamics  method.  Grain  boundaries  were  created  in  U-shaped  model.  Force of boundary surface tension, arising from the boundary intension  to  minimize  its  energy,  was  the  reason  of  directed  movement  of  the  boundary toward its area decrease. The force provoking migration and  migration rate of the boundary remained constant throughout the entire  movement  of  the  boundary,  gradually  decreasing  towards  the  end  of  computer  experiment,  which  made  it  possible  to  measure  migration  rate quite simply. Effect of uniaxial deformation along the X, Y, Z axes  on migration rate of the boundaries was considered. Uniaxial deformation in the model was set at beginning of the computer experiment by  changing  corresponding  interatomic  distances  along  one  of  the  axes.  Interactions of nickel atoms with each other were described with the aid  of Cleri Rosato many-particle potential constructed in the framework  of  tight  binding  model.  For  the  boundaries  considered,  dependences  of  migration  rate  on  misorientation  angle  at  temperature  of  1700 K  were obtained. It is shown that the high-angle <111> and <100> edge  boundaries migrate approximately at the same rate, while mobility of  low-angle  boundaries  differs  significantly:  low-angle  <111>  boundaries migrate about twice as fast as the <100> boundaries. It was found  that in almost all cases, both at elastic compression and tension deformation, migration rate of considered boundaries was slowed down. An  exception was the case of deformation along the <111> edge boundary  axis. When compressing along the edge axis, <111> boundary migrated faster, while on the contrary, it was slower at tension. The obtained  results testify to the fact that migration of edge boundaries is not due to  diffusion processes, such as climbing of dislocations, single migrations of  atoms,  but,  apparently,  by  collective  atomic  permutations:  shifts, slides and splittings of grain boundary dislocations.